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Sunday, July 21, 2013

Curiosity Rover Samples Air for a Taste of Mars History

It’s time to update the
list of ingredients in Martian air.

In late 2012 NASA’s
Curiosity rover drew air into its onboard laboratory and analyzed Mars’s
atmospheric composition with a pair of spectrometers. The results of the
investigation, published July 19 in Science, revise decades-old data on the
makeup of Red Planet air and paint a broad picture of how the atmosphere has
changed since the planet’s formation.

The diaphanous layer of
gas enveloping Mars, where the atmospheric pressure is only about 1 percent of
Earth’s at sea level, is predominantly carbon dioxide, with much smaller
contributionsfrom nitrogen and argon. Together those three gases constitute
more than 99 percent of the atmosphere. (Earth’s atmosphere is also dominated
by three gases: nitrogen, oxygen and argon, in that order, plus a variable amount
of water vapor.)

But NASA’s Viking
mission in the 1970s detected a significantly larger contribution from nitrogen
than argon—2.7 percent nitrogen to 1.6 percent argon. The new measurements from
Curiosity’s Sample Analysis at Mars spectrometers show that the two gases are
present in almost equal amounts. In fact, argon (1.93 percent) appears to be
slightly more prevalent than nitrogen (1.89 percent). The precision of the new
measurements leaves little room for uncertainty, although future data from Curiosity
will help constrain any seasonal variations in the atmospheric makeup.

The rover is already
investigating longer-timescale changes to Mars’s atmosphere by measuring which
gases are present and which nuclear isotopes are in those gases. (Isotopes are
atoms of the same element with different atomic masses.) Researchers can then
compare the isotopic measurements with ancient gases trapped in Martian rocks
that, freed from the surface by impacts, have fallen to Earth as meteorites.

“It tells us for the first
time the atmospheric composition to an accuracy that’s high enough to make
direct comparisons to the meteorites in all our labs,” says experimental
atmospheric scientist Chris Webster of the NASA Jet Propulsion Laboratory in
Pasadena, Calif., lead author of one of the new rover studies.

The ratio of isotopes
in carbon dioxide molecules, for instance, can indicate how much of the Martian
atmosphere has been lost to space—whether blown away by meteor strikes or
stripped by solar wind. Carbon dioxide molecules made with run-of-the-mill
carbon-12 (the most common form of that element) are lighter than molecules
containing the heavy isotope carbon-13 and escape to space easier. Therefore,
atmospheric loss leaves a planet’s carbon dioxide reservoir depleted in
carbon-12 and relatively enriched in carbon-13.

Compared with past
missions such as Viking and NASA’s Phoenix lander, Curiosity has revealed a
greater role for carbon-13, supporting the idea that Mars once had a much
richer atmosphere. Intriguingly, the Curiosity data closely match analyses of a
meteorite called Allan Hills 84001, which formed on ancient Mars before being
ejected into space and subsequently landing in Antarctica. ( The Allan Hills
meteorite rose to fame in 1996, when scientists made the highly controversial
suggestion that it might contain fossilized remains of Mars life.)

“We know that the Allan
Hills meteorite is four billion years old,” Webster says. “It traps gas from
that early Martian atmosphere.” Curiosity, on the other hand, can determine the
precise makeup of the atmosphere today. “So we now have enough confidence and
enough accuracy in the measurements to make that comparison. The overarching
result is that the atmosphere has changed very little in four billion years.”
In other words, it appears the bulk of Mars’s atmosphere was lost relativelyshortly after the planet’s formation 4.5 billion years ago.

That does not mean
there hasn’t been any recent variation. Methane, a gas that some planetary
scientists expect to change greatly over time, is notably absent from the new
studies. In recent years, measurements from Earth have indicated the appearance
anddisappearance of methane plumes on Mars that might spew from geologic—or
even biological—sources. Those observations have stirred controversy, which
Curiosity ought to help settle. The rover has yet to detect the gas, but that
does not necessarily mean it is absent from the Martian atmosphere. The precise
upper limits on methane abundance that rover scientists can infer from Curiosity’s
nondetection will appear in a later study.

“That’s a big story, so
we decided to separate it,” Webster says. “We have a result that’s very
interesting,” he adds, which has been submitted to Science for publication. “We
have no definitive detection of methane—I can tell you that.” It remains to be
seen if Curiosity’s limits on methane abundance strongly conflict with the
levels expected in the presence of seasonal methane belches from the Red
Planet. If they do, the supposed plumes of mysterious origin may be
consigned—alongside the purported fossils in the Allan Hills meteorite—to the
long list of Martian mirages, much to the dismay of optimistic astrobiologistsand an excited public. “It’s not a message people want to hear,” Webster says.
“They don’t really want to hear that there’s no methane on Mars.”

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